A casting assembly consists of a pouring cup, a gating system (including downsprues, choke, and runner), risers, sleeves, molds, cores, and other components. To produce a metal casting, metal is poured into the pouring cup of the casting assembly and passes through the gating system to the mold and/or core assembly where it cools and solidifies. The metal part is then removed by separating it from the core and/or mold assembly.
Risers or feeders are reservoirs which contain excess molten metal which is needed to compensate for contractions or voids of metal which occur during the casting process. Metal from the riser fills such voids in the casting when metal from the casting contracts. Thus the metal from the riser is allowed to remain in a liquid state for a longer period of time, thereby providing metal to the casting as it cools and solidifies. Sleeves are used to surround or encapsulate the riser and other parts of the casting assembly in order to keep the molten metal in the riser hot and maintain it in the liquid state. The temperature of the molten metal and the amount of time that the metal in the riser remains molten is a function of the sleeve composition and the thickness of the sleeve wall, among other factors.
In order to serve their function, sleeves must have exothermic and/or insulating properties. Exothermic sleeves operate by liberating heat thereby keeping the metal hotter and liquid longer. Insulating sleeves, on the other hand, maintain the molten metal in the riser by insulating it from the surrounding mold assembly.
Although aluminosilicate fibers are traditionally used for making insulating sleeves, recently there is an interest in making insulating sleeves using hollow aluminosilicate microspheres, as the insulating material, and an organic binder. The advantage of using hollow aluminosilicate microspheres is that sleeves can be made with better dimensional accuracy than those made with aluminosilicate fibers. Nevertheless, sleeves made with the hollow aluminosilicate microspheres and an organic binder cannot be reused effectively because the organic binder completely degrades at high temperatures (i.e. greater than 500.degree. C.) which are reached during the metal casting process. Once the bonded organic binder degrades, there is nothing left to hold the hollow aluminosilicate microspheres together and the sleeve falls apart.